A similar version of this article appeared in the April 24, 2008 issue of Electronic Design
The circuit of Figure 1
can monitor resistance in a noisy or otherwise hostile environment. Isolation and good common-mode rejection ratio (CMRR) are provided by the transformer (T1).
Figure 1. This circuit monitors the value of a galvanically isolated resistance.
Resistance across the secondary winding is reflected to the primary, where it forms a voltage divider with resistor R1. The divider produces a reduced-amplitude clock signal that is coupled through C2, rectified by D1 and D2, and filtered by C3 and R2. The result, at the inputs of the quad voltage monitor (IC1, MAX6338
) is a dc voltage that varies with the monitored resistance as follows.
|VDC at IC1
The monitored resistance can be in the form of a single resistor, like an NTC resistor for measuring temperature, or a set of resistors and switches as shown. The slow response of the rectifier and filter helps to mask the effect of any switch bounce. The component values shown support a 5V, 0.1MHz-to-1MHz clock signal and resistance values from 0.5kΩ to 10kΩ. IC1's voltage-monitoring thresholds (1.8V, 2.5V, 3.3V, and 5V) are suitable for this application, and T1 can be any 1:1 pulse transformer with an approximate 0.5mH inductance value. You can decode the IC1 output with suitable discrete logic, or feed it to a microcontroller as required.